This invention relates generally to dynamoelectric machines and more particularly to end winding support assemblies of large turbine generators.
During operation of large, hydrogen-inner-cooled generators the temperature of the machine will increase, causing thermal expansion of the various components in the generator end winding assembly. Unless the end winding is totally balanced thermally, or thermal compensation is built into the design, there will be thermal stresses increasing the risk of failure of various components.
The need for allowing for thermal expansion while maintaining the integrity of the end winding assembly has been long recognized. Arrangements to fill that need have been more or less successful but have generally left a continuing desire for alternative arrangements and methods of assembly that are both effective and inexpensive to practice. A further desirable quality is the ability to retrofit an improved thermal compensation support assembly in the end windings of an existing machine in the field. The present invention addresses these concerns in providing a new end turn assembly and method of construction that is very effective and easily applicable to new machine construction. A significant element of the new assembly can also be installed in an existing machine in the field without requiring removal of the coils.
By the present invention, two significant features are incorporated in the end winding assembly for new or rewound machines. One feature to consolidate the end winding is the use of keyed strain blocks mounted between top coils and bottom coils at several locations. The strain blocks are shaped to fit tightly between the coils. They support the top coils and transfer electromagnetic forces from the top coils to the coil support bracket that is affixed to the stator core.
During operation, the top and bottom coils expand due to the thermal expansion. Some of the expansion takes place in the axial direction of the coil involute creating a torque on the strain block that may overstress the bond between the strain blocks and the coils, as strain blocks have been previously used. Former designs on which the present invention improves utilize strain blocks mounted flush to a middle coil support ring with merely a single abutting surface. By the present invention, the strain blocks are keyed or interlocked with the support ring. By keying the strain block to the thermally matched middle coil support ring, the force created by the torque can partly be taken up by the strain block and the stress between the strain block and the coil will decrease since only shear stress and no tensile stress will occur in the bond. The keyed strain block is also preferably long enough in the axial direction to overlap four to six top and bottom coils for further reduction of the shear stresses. By referring to the strain blocks as "keyed" is meant that they are physically fit within notches in the coil support ring which is preferable to using pins or other types of fasteners that increase assembly time.
The other principal feature of the invention, and one which can be applied to machines in the field without requiring rewinding, as well as in new manufacture, is a new form of brace. During operation of the generator, the stator coils expand axially out of the core creating high stresses between the bottom coils and the coil support brace utilized as an intermediate element between the coils and the bracket joined to the core. By the present invention, the brace comprises a plurality of rigid pieces including at least a top and a bottom piece between which is an intermediate decoupler that includes a layer of material such as rubber for cushioning between the elements and an antifriction layer such as polytetrafluoroethylene for low friction. The system will allow for free expansion in the axial direction because of the low friction layer. This eliminates thermal stress and prevents failure of the bond between coils and braces thereby eliminating wear of the coil at this interface. The brace will continue to function as a stop to prevent excessive coil deflection during short circuits. The rubber layer will allow for resilient absorption during an impact in the radial direction. The top piece is bonded to the bottom coil such as by using epoxy-impregnated Dacron fabric material for conformability. This will allow for various coil shapes. The top piece of the brace is also attached to the bottom coil support ring to increase the integrity of the system. The top piece also serves as extra protection for the bottom coil insulation.
Besides allowing for thermal expansion, this brace design will dynamically uncouple the end winding from the core minimizing the core influence on the end winding vibration. Any relative motion, either thermal or vibratory, must occur at the low friction interface, rather than at the coil insulation surface.